Very nice gun sir. According to Suprica's book, "Very early production may have a small lock screw on the upper side plate screw."
It also says that a nickeled one is valued at twice that of a blued one.
So it sounds like not to many of them were made with the lock screw which yours has. How many is anyone's guess but some one here may have a better idea.
Got a magnet handy? Any chance that yours has the aluminum cylinder?